Remove K&R vestiges.

* configure.ac (AC_C_CONST, AM_C_PROTOTYPES): Remove.
* src/complain.c (VA_START): Remove.  Assume prototypes.
(vfprintf) [! (HAVE_VPRINTF || defined vfprintf)]: New macro.
(private_strerror, warn_at, warn, complain_at, complain, fatal_at,
fatal): Assume prototypes.
* src/complain.h: Assume prototypes.
* src/system.h (PARAMS): Remove.
Include <limits.h> unconditionally, since it's guaranteeed even
for a freestanding C89 compiler.
(SHRT_MIN, SHRT_MAX): Remove, since C89 guarantees them.
* src/vmsgetargs.c (getargs, cli_present, cli_get_value): Prototype.