Rename pg_rewind's copy_file_range() to avoid conflict with new linux syscall.

Upcoming versions of glibc will contain copy_file_range(2), a wrapper
around a new linux syscall for in-kernel copying of data ranges. This
conflicts with pg_rewinds function of the same name.

Therefore rename pg_rewinds version. As our version isn't a generic
copying facility we decided to choose a rewind specific function name.

Per buildfarm animal caiman and subsequent discussion with Tom Lane.

Author: Andres Freund
Discussion:
    https://postgr.es/m/20180103033425.w7jkljth3e26sduc@alap3.anarazel.de
    https://postgr.es/m/31122.1514951044@sss.pgh.pa.us
Backpatch: 9.5-, where pg_rewind was introduced