RFR: 6173: jmc -open requires full path

[Jean-Philippe Bempel]

Handle special Mac case where user.dir is set by mac eclipse launcher
to the directory where is jmc binary.
Hopfully, PWD env var is kept where the program was launched so we can use it
to prepend to the providing filename.

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Commit messages:
 - 6173: jmc -open requires full path

Changes: https://git.openjdk.java.net/jmc/pull/123/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=123&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6173
  Stats: 16 lines in 1 file changed: 14 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/jmc/pull/123.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/123/head:pull/123

PR: https://git.openjdk.java.net/jmc/pull/123