http::FsHandler: take advantage of efficient copy between file descriptors
http::FsHandler: take advantage of efficient copy between file descriptors

See that we use sendfile to send file over http:

[pid  5081] read(6, "GET /data/include/unistd.h HTTP/"..., 65536) = 135
[pid  5081] stat("/usr/include/unistd.h", {st_mode=S_IFREG|0644, st_size=42983, ...}) = 0
[pid  5081] mprotect(0x7fbdd4021000, 4096, PROT_READ|PROT_WRITE) = 0
[pid  5081] open("/usr/include/unistd.h", O_RDONLY) = 8
[pid  5081] writev(6, [{"HTTP/1.1 200 OK\r\nServer: mimosa\r"..., 138}], 1) = 138
[pid  5081] fstat(8, {st_mode=S_IFREG|0644, st_size=42983, ...}) = 0
[pid  5081] sendfile(6, 8, NULL, 42983) = 42983
[pid  5081] close(8)                    = 0
[pid  5081] read(6, "", 65536)          = 0
[pid  5081] close(6)                    = 0

abique/mimosa,abique/mimosa