POSIX xgettext and the initial domain directive

[Bruno Haible]

https://posix.rhansen.org/p/gettext_draft
Line 1183

"The first directive in each created dot-po file shall be a domain directive
giving the associated domain name"

GNU gettext currently does not do this. Solaris gettext does it.
The msgfmt program allows the initial domain directive to be absent
(see lines 996-998).

What is the added value of this directive, since for msgfmt it is optional?
In 99% percent of the cases, xgettext is used as part of a build system for
a single domain. The author of that build system knows the domain perfectly
well. There is no need to additionally store it in the dot-po file.

Issue: Since this directive was not documented in
https://www.gnu.org/software/gettext/manual/html_node/PO-Files.html
many PO file consumers will choke on this directive, once GNU xgettext
implements the POSIX specification.

Suggestion: Declare that it is implementation-dependent whether xgettext
writes out a domain directive, when the output contains only entries for a
single domain.