eupspkg.sh: clear some envvars to avoid accidental interference
eupspkg.sh: add EUPSPKG_ prefix to variables inherited from the environ.

Currently, this applies to only three:

	EUPSPKG_REPOSITORY_PATH
	EUPSPKG_SOURCE
	EUPSPKG_VERBOSE

When specified on the command line, no EUPSPKG_ prefix should be used (i.e.,
eupspkg SOURCE=...  <verb>)
eupspkg.sh: support VERSION_PREFIX and VERSION_SUFFIX

$VERSION_PREFIX and $VERSION_SUFFIX are the prefix/suffix that
version_to_gitrev() removes from version before interpreting it as a git
rev (to be interpretable by git-rev-parse).

This lets one have a such as:

	hsc_1.2.3_c

that is converted into a 1.2.3 rev (which will, most likely, be a tag).

Also, when in DEVMODE autoversion() will prepend/append the _PREFIX, _SUFFIX
to the autogenerated version.

Environment variables EUPSPKG_VERSION_PREFIX and EUPSPKG_VERSION_SUFFIX will
be consulted if VERSION_{PREFIX|SUFFIX} are left unspecified.
eupspkg.sh: non-intrusive site-specific customization of eupspkg scripts

The package distributor can have eupspkg.sh source a list of shell scripts
specified in a :-delimited string in SITECUSTOMIZE keyval argument (or
EUPSPKG_SITECUSTOMIZE environment variable).