Better defaults for external tools

[gaborcsardi]

• EDITOR -- set to $VISUAL or vi, which seems ok. This is a personal choice, so no obvious better default here.
• MAKE -- the make default should be fine on all Linux
• Better default for PAGER #25
• Better default for R_BROWSER #26
• R_BZIPCMD -- /bin/bzip2, seems OK
• R_GZIPCMD -- /bin/gzip, seems OK
• Better default for R_PDFVIEWER #27
• Better default for R_PRINTCMD #28
• R_QPDF -- empty, not terribly important?
• R_RD4PDF -- fonts to use for Rd2pdf, seems fine
• R_TEXI2DVICMD -- does not seem important
• Better default for R_UNZIPCMD #29
• Better default for R_ZIPCMD #30
• SED -- /bin/sed, seems fine
• TAR -- /bin/tar, seems fine