Mercurial Merge Request #2: Fix compilation with -Werror=strict-prototypes (merged)

An upcoming release of clang-16 will enable strict prototype checking by default.
In Gentoo Linux (link) I found that joe currently has a few instances where old-style prototypes are used; this MR fixes them. It also fixes an incorrect use (redefinition) of errno. These changes should not affect portability on other platforms or compilers.
I apologize in advance if this MR somehow does not follow regular branching conventions - it's been 14 years since I last used mercurial.
Thank You for joe!

[John J. Jordan]

Looks good.