Rename net.{c,h} to net_sockets.{c,h}

The library/net.c and its corresponding include/mbedtls/net.h file are
renamed to library/net_sockets.c and include/mbedtls/net_sockets.h
respectively. This is to avoid naming collisions in projects which also
have files with the common name 'net'.
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index f40d52f..afbcaff 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -332,7 +332,7 @@
 scripts/config.pl unset MBEDTLS_SSL_CLI_C
 CC=gcc CFLAGS='-Werror -O0' make
 
-msg "build: full config except net.c, make, gcc -std=c99 -pedantic" # ~ 30s
+msg "build: full config except net_sockets.c, make, gcc -std=c99 -pedantic" # ~ 30s
 cleanup
 cp "$CONFIG_H" "$CONFIG_BAK"
 scripts/config.pl full