Removed inclusion of getopt.h from argp.h: we aren't going to have a public getopt.h. Nor do we need one to get argp.h through a compiler.
argp-eexst.c: Use sysexits.h and EX_USAGE if available, use stdlib.h and EXIT_FAILURE otherwise.
argp-fmtstream.h: Only include unistd.h if available. Added fix for missing ssize_t when compilding with MSC.
argp-namefrob.h: only include headers if they're present according to config.h
argp-help.c: include stdint.h and stdbool.h to get uint8_t and bool