libinterimap: use directories relative to $HOME for the XDG defaults.

libinterimap: use directories relative to $HOME for the XDG defaults.

Previously getpwuid() was called to determine the user's home directory,
while the XDG specification explicitely mentions $HOME.

Conveniently our docs always mentioned ~/, which on POSIX-compliant
systems expands to the value of the variable HOME (and the result is
unspecified when the variable is unset).  Cf. Shell and Utilities volume
of POSIX.1-2017, sec. 2.6.1:

    https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_01