man: use singular “they”

man: use singular “they”

For an example where we already use it, see man:sd-login(3):

> A session is defined by the time a user is logged in until they log out.

As far as I can tell, this removes the only remaining occurrences of
referring to users by gendered pronouns in our documentation (though
some still survive in code comments and the NEWS and TODO files):

    git grep '\b\(he\|him\|his\|she\|her\|hers\)\b' man/