libmount: fix const qualifier warnings for C23

libmount: fix const qualifier warnings for C23

Fix const qualifier discarded warnings in optlist_add_flags(),
mnt_opt_value_with(), and mnt_optstr_apply_flags() functions.
These warnings are reported by gcc 15 which defaults to the C23 standard.

The strchr() and strstr() functions return pointers into const strings,
so the receiving variables must be declared as const char *.

Signed-off-by: Karel Zak <kzak@redhat.com>