net/textproto: reduce allocations in ReadMIMEHeader
net/textproto: reduce allocations in ReadMIMEHeader

ReadMIMEHeader is used by net/http, net/mail, and
mime/multipart.

Don't do so many small allocations. Calculate up front
how much we'll probably need.

benchmark                  old ns/op    new ns/op    delta
BenchmarkReadMIMEHeader         8433         7467  -11.45%

benchmark                 old allocs   new allocs    delta
BenchmarkReadMIMEHeader           23           14  -39.13%

benchmark                  old bytes    new bytes    delta
BenchmarkReadMIMEHeader         1705         1343  -21.23%

R=golang-dev, r, iant, adg
CC=golang-dev
https://codereview.appspot.com/8179043

sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,mhennings/marcohennings-go,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,rdp/rogerpack2005-golang,webfd/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,mhennings/marcohennings-go,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,webfd/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,d0f/go-zh,mhennings/marcohennings-go,bryanxu/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof