Make various scripts work when HOME is unset

Use tilde expansion for the bash scripts and completion.

Use File::HomeDir->my_home for the Perl scripts.
Store results in $ENV{HOME} to minimise the changes.

The scripts that had issues when HOME is unset include:

bts
chdist
debcommit
dscverify
grep-excuses
namecheck
rc-alert
svnpath
wnpp-alert