gdb/cli: Factor out code to list lines around a given line

A future patch will add more situations that calculates "lines around a
certain point" to be printed using print_source_lines, and the logic
could be re-used. As a preparation for those commits, this one factors
out that part of the logic of the list command into its own function.
No functional changes are expected

Approved-By: Tom Tromey <tom@tromey.com>