problem with last change to _get_pids_by_name() in pcp.env

[Ken McDonell <kenj@xxxxxxxxxxxxxxxx>]

Commit 63ce703 forces matching to the full pathname of the binary (not a 
bad idea in principle, and motivated by a real issue) ... but the 
implementation fails (spectacularly) when the
(a) the shell running _get_pids_by_name() has a different $PATH than the 
shell that was used to launch the binary in question, so which(1) and 
exec(2) may refer to different binaries, and/or
(b) the binary in question was launched via a symbolic link.

Specifically pmlogger (on at least some distros) lives in /usr/bin and 
as a symlink in $PCP_BINADM_DIR ... I have a bunch of QA failures from 
the resulting mismatch.
I don't have a fix as yet, but this is a heads up for others pending 
resolution.