docs/defines.txt: document _extern_inline

docs/defines.txt: document _extern_inline
libc/inet/ether_addr.c: optimize tolower, we ever use it on 0-9a-f
libc/inet/ntop.c: optimize tolower, we ever use it on 0-9a-f
 with locale support on, code size change is:

    text           data     bss     dec     hex filename
-    396              0      24     420     1a4 libc/inet/ether_addr.os
+    306              0      24     330     14a libc/inet/ether_addr.os
-   1453              0       0    1453     5ad libc/inet/ntop.os
+   1430              0       0    1430     596 libc/inet/ntop.os