Update the PCP platform to use common Process fields and code

Remove code now that we have common platform-independent command
line wrangling (thanks BenBE!).  Add PCP platform support for a
handful of other recently arriving odds and ends - ELAPSED time,
CWD, and so on.

pcp/Platform.h

@@ -206,6 +206,8 @@ typedef enum Metric_ {
    PCP_PROC_LABELS,             /* proc.psinfo.labels */
    PCP_PROC_ENVIRON,            /* proc.psinfo.environ */
    PCP_PROC_TTYNAME,            /* proc.psinfo.ttyname */
+   PCP_PROC_EXE,                /* proc.psinfo.exe */
+   PCP_PROC_CWD,                /* proc.psinfo.cwd */
 
    PCP_PROC_ID_UID,             /* proc.id.uid */
    PCP_PROC_ID_USER,            /* proc.id.uid_nm */