Make third person singular pronouns gender neutral

[ecawthon]

Various places in both documentation and default emails to admins/moderators
use he/him/his to refer to generic users of unknown gender. This commit
changes all generic uses of male pronouns to use singular they/them/their.
It also changes "his/her" to "their", but leaves "he or she" unchanged due
to less trivial conjugation issues. NEWS, AUTHORS.md, and OChangeLog are
unchanged.

Motivating example:

WARNING: alice@example.com failed to unsubscribe from example-list because
his address was not found in the list.

becomes

WARNING: alice@example.com failed to unsubscribe from example-list because
their address was not found in the list.

[ikedas]

Hi @ecawthon,
Thanks for precise work! I'll review your corrections and include them into current source.

---

Note: If you would make corrections on strings to be translated (msgid) in the future, you may only change en_US.po files which contain all strings in .tt2 files, several messages in .pm files and all other .po files. If en_US.po was updated, corrections will be included in corresponding files at the next time of release.

To add correction to en_US.po:

1. In an en_US.po file, go to a msgid (string to be translated), and copy the content of it to content of msgstr (result of translation).
2. Remove a line #, fuzzy above, if any.
3. Correct content of msgstr.

Or, using English page on Translation site is easier:

[ikedas]

Merged. However @ecawthon, I worry if I couldn't sync your corrections enough with update on translation site. If possible, could you please check translate.sympa.org.

Thanks for big work!