Fix png filename handling

Fix png filename handling

    This patch can compile the source file
    whose name contains `%' and convert to png files.
    (e.g. `%foobar.ly' to `%foobar.png')

Use ly:system in make-ps-images (ps-to-png.scm)

    In make-ps-images,
    use ly:system instead of my-system for invoking gs.
    It is the same way as postscript->pdf
    (backend-library.scm).

Delete duplicate procedures in ps-to-png.scm

    PLATFORM: lily.scm
    search-executable: backend-library.scm
    search-gs: backend-library.scm
    gulp-port: unused

[trueroad, 2015-05-24 15:08:01]

I've found that LilyPond fail to compile the source file whose name contains `%'
to png files.
For example, the following command fails.

$ lilypond --png %foobar.ly

This patch solve it.

[pkx166h, 2015-05-31 11:00:06]

Patch counted down - please push

[trueroad, 2015-05-31 13:46:55]

I've pushed

Issue 4417 / 3: Fix png filename handling

author    Masamichi Hosoda <trueroad@trueroad.jp>
          Sun, 24 May 2015 14:20:30 +0000 (23:20 +0900)
committer Masamichi Hosoda <trueroad@trueroad.jp>
          Sun, 31 May 2015 13:27:28 +0000 (22:27 +0900)
commit    6a59207988b737482012aee97c6d3d5c1458a750

Issue 4417 / 2: Use ly:system in make-ps-images (ps-to-png.scm)

author    Masamichi Hosoda <trueroad@trueroad.jp>
          Sun, 24 May 2015 13:14:53 +0000 (22:14 +0900)
committer Masamichi Hosoda <trueroad@trueroad.jp>
          Sun, 31 May 2015 13:27:27 +0000 (22:27 +0900)
commite   951c7975ad91c03e52856b954206d83fa04dcb1

Issue 4417 / 1: Delete duplicate procedures in ps-to-png.scm

author    Masamichi Hosoda <trueroad@trueroad.jp>
          Sun, 24 May 2015 05:28:43 +0000 (14:28 +0900)
committer Masamichi Hosoda <trueroad@trueroad.jp>
          Sun, 31 May 2015 13:27:27 +0000 (22:27 +0900)
commit    42494c126671d9b70f7274ef502c9dc33e72177f