usr.bin/xlint: convert to SRCTOP idioms
The former idioms used were .CURDIR-relative paths. Using SRCTOP (as
discussed previously), makes pathing more intuitive, abbreviates make
output by resolving relative paths absolute to the top of the source
tree, makes portions of the tree relocatable/forkable, etc.
Use XLINT_SRC to refer to ${SRCTOP}/usr.bin/xlint in a shorthand manner.
MFC after: 1 month
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