Declare load_hosts() as returning HostsFileLoadResult.

Declare load_hosts() as returning HostsFileLoadResult.

This function returns some value of enum HostsFileLoadResult,
but for reasons lost in the development process was declared to
return "int".  Fix that, for clarity and so that our typedefs
collection tooling sees the typedef as used.  Also fix the
variable that the sole call assigns into.  Move the typedef
to the header file that declares load_hosts() to avoid creating
header dependency problems.

Discussion: https://postgr.es/m/359138.1777922557@sss.pgh.pa.us