GCC 15 tightened its built-in declarations for strchr() and strstr() so
that they now propagate const from their first argument, triggering
-Werror=discarded-qualifiers on three assignments in string.c:
translate():
char *match = strchr(from, *pos)
`from` is const char *, so the result of strchr() is const char *.
`match` is only used for pointer arithmetic (match - from), so
declaring it const char * is correct and safe.
strreplace():
found = strstr(str, search) [line ~73]
found = strstr(pos, search) [line ~89, while condition]
`str`/`pos` are derived from a const char * parameter, so strstr()
returns const char *. `found` is used as a mutable char * later
(pos = found + slen), consistent with the existing (char*) casts
already used throughout this function for the same reason.
Add explicit (char*) casts to match the established pattern.
Closesstrongswan/strongswan#3015
