Integer type policy

Integers are for numbers, enabling arithmetic like subtractions and for
loops without getting shot in the foot. Unsigneds are for bitfields.

- "int" for numbers that will always be laughably smaller than four
  billion, and where we don't care about the serialization format.

- "int32" for numbers that will always be laughably smaller than four
  billion, and will be serialized to four bytes.

- "int64" for numbers that may approach four billion or will be
  serialized to eight bytes.

- "uint32" and "uint64" for bitfields, depending on required number of
  bits and serialization format. Likewise "uint8" and "uint16", although
  rare in this project since they don't exist in XDR.

- "int8", "int16" and plain "uint" are almost never useful.

cmd/syncthing/memsize_linux.go

@@ -23,7 +23,7 @@ import (
 	"strings"
 )
 
-func memorySize() (uint64, error) {
+func memorySize() (int64, error) {
 	f, err := os.Open("/proc/meminfo")
 	if err != nil {
 		return 0, err
@@ -40,7 +40,7 @@ func memorySize() (uint64, error) {
 		return 0, errors.New("/proc/meminfo parse error 2")
 	}
 
-	kb, err := strconv.ParseUint(fs[1], 10, 64)
+	kb, err := strconv.ParseInt(fs[1], 10, 64)
 	if err != nil {
 		return 0, err
 	}