Missing/misplaced MVS prototypes: loadenv, tzset, __exit, __svc99

[mgrossmann]

Several libc370 runtime routines are linkable on MVS but have no usable public prototype, so consumers get implicit-declaration warnings and must declare them locally. Surfaced while building httplua (mvslovers/httplua, worked around in commit a804a8f).

Functions with no public header

Function	Suggested header	Notes
int loadenv(const char *ddname)	clibenv.h	Loads env vars from a DD (e.g. dd:SYSENV); used by CGI programs.
void tzset(void)	time.h	POSIX; time.h currently declares time/localtime/mktime/etc. but not tzset.
void __exit(int rc)	clibcrt.h (or stdlib.h)	Runtime exit.

__svc99 is guarded under #ifdef MUSIC

mvssupa.h declares int __svc99(void *rb); only inside #ifdef MUSIC:

#ifdef MUSIC
int __system(...);
int __textlc(void);
int __svc99(void *rb);
#else
int __system(int req_type, size_t pgm_len, char *pgm, size_t parm_len, char *parm);
...
#endif

SVC 99 (dynamic allocation) is an MVS service, so on MVS (the #else branch)
the prototype is missing. It should be available outside the MUSIC guard.

Impact

The functions link fine (libc370 defines them); only the prototypes are absent,
which trips -Wimplicit-declaration in consumers. Adding the prototypes lets
httplua (and any future consumer) drop its local extern declarations.