[patch] give alternates for read only variable warning

[Stan Cox]

(Tweaked to return a slightly different format warning)

Given the same example as before:

WARNING: read-only local variable identifier 'elide_me5'
at /work/scox/systemtap/src/testsuite/systemtap.base/warnings.stp:7:31
(alternatives:  elide_me4 elide_me1)
WARNING: read-only local variable identifier 'elide'
at /work/scox/systemtap/src/testsuite/systemtap.base/warnings.stp:8:21
(alternatives:  me1 elide_me1)
WARNING: read-only local variable identifier 'me1'
at /work/scox/systemtap/src/testsuite/systemtap.base/warnings.stp:8:27
(alternatives:  elide elide_me1)
WARNING: read-only local variable identifier 'var'
at /work/scox/systemtap/src/testsuite/systemtap.base/warnings.stp:14:54
(alternatives:  elide_me6 elide_me1)
WARNING: read-only local variable identifier 'elide'
at /work/scox/systemtap/src/testsuite/systemtap.base/warnings.stp:6:24
(alternatives: me1 elide_me1)
WARNING: read-only local variable identifier 'me1'
at /work/scox/systemtap/src/testsuite/systemtap.base/warnings.stp:6:30
(alternatives: elide elide_me1)
WARNING: read-only local variable identifier 'b'
at /work/scox/systemtap/src/testsuite/systemtap.base/warnings.stp:17:27
(alternatives: a elide_me7 elide_me1)