These POSIX functions are declared in strings.h, so include this
header explicitly instead of relying on the libc's default feature-test
macros to include it through string.h.
All programs except mshow have a very tight set of promises. mshow
has a broad set of promises and might be a good future candidate
to further restrict using unveil(2).
This patch is based on commit 0300a112 by Alex Holst (dated
2017-12-07), which was proposed in GH PR #79.
* pledged mpick, mflow and mdate so that now all programs are pledged
* removed some unneeded promises and added some missing promises
* move err.h include and OpenBSD ifdef into a new xpledge.h
* cleaned up code aligning and whitespace
Closes: #179 [via git-merge-pr]
