Have programs set the program name, at the beginning of main(), with
g_set_prgname().  That lets other code call g_get_prgname() to get the
program name.

This lets us replace a bunch of xxx_cmdarg_err() and
xxx_cmdarg_err_cont() routines with common code, as they differ only in
the command name printed in error message, and the common routine can
use g_get_prgname().

This also means that ws_log_init() doesn't need to be passed the
program name; it can juse use g_get_prgname().  The same applies to
extcap_log_init(), which calls ws_log_init().

While we're at it, have a common routine for extcaps to use ws_logv() to
log warnings, replacing some other xxx_cmdarg_err() and
xxx_cmdarg_err_cont() routines.

Use g_get_prgname() in ws_getopt.c; this means that you don't run the
risk of a pathname, rather than a command name, show up in the error
message.