libc: rewrite putenv and getenv.

These new implementations

- do several validity check on input parameters
- allow a bit larger variable names (127 bytes, aka sizeof(Proc.genbuf)-1)
- preserve nulls in the content (the original version used to replace
  '\0' with ' '). I can't see why they did, actually.
  See also http://marc.info/?l=9fans&m=148475801229908&w=2

Should also fix CID 155718

sys/include/libc.h

@@ -46,7 +46,7 @@ extern	int	strncmp(const char*, const char*, int32_t);
 extern	char*	strpbrk(const char*, const char*);
 extern	char*	strrchr(const char*, int);
 extern	char*	strtok(char*, char*);
-extern	int32_t	strlen(const char*);
+extern	int	strlen(const char*);
 extern	int32_t	strspn(const char*, const char*);
 extern	int32_t	strcspn(const char*, const char*);
 extern	char*	strstr(const char*, const char*);