Get a warning-less compilation to use -Werror with travis

[illwieckz]

To not let contributors introduce warnings (like “integer from pointer”), we must use -Werror at compilation time when someone submits a commit (#79), but before that, we must clean all of those already existing:

gcc said that:

pref.c:3742:6: warning: variable ‘val’ set but not used [-Wunused-but-set-variable]
pref.c:3748:6: warning: variable ‘val’ set but not used [-Wunused-but-set-variable]
skin.c:285:6: warning: pointer targets in assignment differ in signedness [-Wpointer-sign]
skin.c:310:6: warning: pointer targets in assignment differ in signedness [-Wpointer-sign]
srv-prop.c:691:6: warning: suggest parentheses around operand of ‘!’ or change ‘&’ to ‘&&’ or ‘!’ to ‘~’ [-Wparentheses]
srv-prop.c:691:43: warning: suggest parentheses around operand of ‘!’ or change ‘&’ to ‘&&’ or ‘!’ to ‘~’ [-Wparentheses]
xqf-ui.c:365:42: warning: operation on ‘(cldef->cols + (sizetype)((long unsigned int)column * 56ul))->current_sort_mode’ may be undefined [-Wsequence-point]

[illwieckz]

About skin.c, see #39.

[illwieckz]

clang said:

filter.c:524:27: warning: comparison of unsigned expression < 0 is always false [-Wtautological-compare]
        if (current_server_filter<0 || current_server_filter>server_filters->len)
filter.c:1183:28: warning: comparison of unsigned expression < 0 is always false [-Wtautological-compare]
        if (current_server_filter < 0) {
skin.c:285:6: warning: assigning to 'guchar *' (aka 'unsigned char *') from 'char *' converts between pointers to integer types with different sign [-Wpointer-sign]
        pcx = read_skin_pcx (filename, FALSE);
skin.c:310:6: warning: assigning to 'guchar *' (aka 'unsigned char *') from 'char *' converts between pointers to integer types with different sign [-Wpointer-sign]
        pcx = read_skin_pcx (filename, TRUE);
xqf.c:3273:3: warning: initializing 'gpointer' (aka 'void *') with an expression of type 'const struct menuitem (*)[4]' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
                &file_menu_items,
xqf.c:3282:3: warning: initializing 'gpointer' (aka 'void *') with an expression of type 'const struct menuitem (*)[18]' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
                &edit_menu_items,
xqf.c:3291:3: warning: initializing 'gpointer' (aka 'void *') with an expression of type 'const struct menuitem (*)[7]' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
                &view_menu_items,
xqf.c:3300:3: warning: initializing 'gpointer' (aka 'void *') with an expression of type 'const struct menuitem (*)[12]' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
                &server_menu_items,
xqf.c:3309:3: warning: initializing 'gpointer' (aka 'void *') with an expression of type 'const struct menuitem (*)[10]' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
                &preferences_menu_items,
xqf.c:3327:3: warning: initializing 'gpointer' (aka 'void *') with an expression of type 'const struct menuitem (*)[2]' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
                &help_menu_items,