pgsql: Fix our getopt_long's behavior for a command line argument of ju

Fix our getopt_long's behavior for a command line argument of just "-".

src/port/getopt_long.c failed on such an argument, always seeing it
as an unrecognized switch.  This is unhelpful; better is to treat such
an item as a non-switch argument.  That behavior is what we find in
GNU's getopt_long(); it's what src/port/getopt.c does; and it is
required by POSIX for getopt(), which getopt_long() ought to be
generally a superset of.  Moreover, it's expected by ecpg, which
intends an argument of "-" to mean "read from stdin".  So fix it.

Also add some documentation about ecpg's behavior in this area, since
that was miserably underdocumented.  I had to reverse-engineer it
from the code.

Per bug #16304 from James Gray.  Back-patch to all supported branches,
since this has been broken forever.

Discussion: https://postgr.es/m/16304-c662b00a1322db7f@postgresql.org

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5f9eace5db4ef9d020aef44f972cea627b663932

Modified Files
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doc/src/sgml/ref/ecpg-ref.sgml | 31 ++++++++++++++++++++++++++-----
src/port/getopt_long.c         | 14 +++++++++++---
2 files changed, 37 insertions(+), 8 deletions(-)