move accept4, dup3, and pipe2 to non-linux-specific locations

these interfaces have been adopted by the Austin Group for inclusion
in the next version of POSIX.

diff --git a/src/linux/accept4.c b/src/network/accept4.c
similarity index 100%
rename from src/linux/accept4.c
rename to src/network/accept4.c

diff --git a/src/linux/dup3.c b/src/unistd/dup3.c
similarity index 100%
rename from src/linux/dup3.c
rename to src/unistd/dup3.c

diff --git a/src/linux/pipe2.c b/src/unistd/pipe2.c
similarity index 100%
rename from src/linux/pipe2.c
rename to src/unistd/pipe2.c