man pmlookupdesc.3: correct pmUnits documentation

The scaleSpace and scaleTime have been unsigned:4 bitfields since
commit 1c5cb90d01 back in 2009.  Correcting the documentation to match.
PMAPI extension: pmParseUnitsStr

We introduce a function that reverses pmUnitsStr (and then some), so
that users can specify desired metric unit/scale conversions in
somewhat free-form textual form.  The API function looks like this:

     int pmParseUnitsStr(const char *, pmUnits *, double *)

where the latter two arguments are the results.  The first output is
what one would expect.  The last parameter is a final arithmetic
multiplier, which may be simply 1.0 for a straight pmUnitsStr() format
text, or is the appropriate factor to compensate for a user asking for
conversion of a quantity to

     "10 megabytes / 15 minutes"
qa for new pmParseUnitsStr() function

The tests include an exhaustive scan, round-tripping all valid
pmUnits() tuples through pmUnitsStr and back through pmParseUnitsStr.
("valid" excludes tuples with dim=0 but scale!=0.  Also, due to a
parsing ambiguity with count scale-vs-dimensions, the round-tripping
check is slightly adjusted for them.)

The 670 test also exhaustively tests all new keywords, and all
forcesable code paths (as guided by gcov output).  Floating point
calculations are all in double but rendered to default %g precision,
so should be accurate across platforms.  (Tested on linux x86-64).
man page for new pmParseUnitsStr() function
demo for pmParseUnitsStr: procmemstat.c

This little demo program now takes an optional -u "STRING", which lets
a user rescale numbers to units other than 'kbytes'.  The code now
also demonstrates nontrivial command line option handling patterns.
add errMsg output for pmParseUnitsStr() function

Other pmParse* functions produce some descriptive text in case
of a parse error.  Let's start following suit.
pmParseUnitsStr whitespace work

Processed through GNU indent, with options approximating the pcp style:

 -nhnl -ut -br -i4 -il0 -cli4 -npcs -cbi0 -l120 -saf -sai -saw -cs
pmParseUnitsStr review tweaks

Handling issues as per brolley's review:
http://oss.sgi.com/pipermail/pcp/2014-December/006185.html
pmParseUnitsStr: switch from c99 "//" comments to c72 "/* */".
Merge branch 'fche/units-parse' of ../../pcpfans/fche/units-parse into dev

Conflicts:
	src/libpcp/src/exports