These changes are the result of some regression testing, plus a top
to bottom read through prompted by a user reporting bug #1 below.
Fortunately I only found that one additional instance of forgetting
to virtualize a value we derive from bsd.port.mk, but I did find and
fix a few more things as well.

Bug Fixes

1. Virtualize an occurrence of $distdir instead of using $pd/distfiles
2. Make all variables in read_distinfos() local
3. When specifying multiple ports on the command line, the test for

PARENT_PID to start read_distinfos() would always fail, so start it
in the initial PARENT_PID block instead. This also implies pulling the
DI_FILES creation up into that same block.

Non-Functional Changes

1. Adjust whitespace in the code, and the script output in a few places
2. Use a shell expansion instead of sed to trim multiple results
3. Remove one redundant error check
4. Replace the name find_contents_distfiles() with find_dl_distfiles()

since the info is not in +CONTENTS anymore

5. Add/update comments to the existing code

Small changes to actual behavior

1. When dealing with unrecorded dependencies pipe the results to

'less -F'

2. Minor optimization: In read_distinfos(), check to see if we're

upgrading an existing port, and if so do that port first so we're
sure the right distfiles will be in DI_FILES even if the function
doesn't complete before the port is done building/installling

3. Add a message to -L to indicate when there are no new versions
4. Move the checks for run-depends after installation of the parent

port so that if a run-depends port has a build-depends on the parent,
it will work. This is a supported, but apparently not recommended
configuration. To make this work do the install with -DNO_DEPENDS so
that we can take charge of the run-depends installs.