Fix the broken default value handling in mew-input-range

This commit fixes a defect in my previous commit 4af7048 which broke the
case where askp is nil.
change the image resizing command

pnmscale was obsoleted by pamscale.
change the image converting command

pngtopnm was obsoleted by pngtopam.
pnmtotiff was extended and renamed to pamtotiff.
pnmtogif is not present but pamtogif is.
Merge pull request #42 from tadanagao/completion-defaults-001

Fix the broken default value handling in mew-input-range
Merge pull request #44 from trueroad/patch-2

change the image converting command
Merge pull request #43 from trueroad/patch-1

change the image resizing command
Update mew-mime-content-type to support xz suffixes
Merge pull request #45 from tats/bug/xz-content-type

Update mew-mime-content-type to support xz suffixes
Fix a comment line row position

The comment line row position is wrong.
Merge pull request #46 from trueroad/patch-3

Fix a comment line row position
Add some Content-Types

image/bmp: Windows bitmap (*.bmp), unofficial, IANA unregistered, Microsoft's de facto standard.
image/jpg: JPEG (*.jpg), unofficial, IANA unregistered, some MUA's typo, registered type is image/jpeg.
image/vnd.microsoft.icon: Windows icon (*.ico), IANA registered.
Merge pull request #47 from trueroad/patch-4

Add some Content-Types