global: improve const correctness when assigning string constants

We're about to enable `-Wwrite-strings`, which changes the type of
string constants to `const char[]`. Fix various sites where we assign
such constants to non-const variables.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/regex/regcomp.c

@@ -848,7 +848,7 @@ init_dfa (re_dfa_t *dfa, size_t pat_len)
 {
   unsigned int table_size;
 #ifndef _LIBC
-  char *codeset_name;
+  const char *codeset_name;
 #endif
 
   memset (dfa, '\0', sizeof (re_dfa_t));