core.sysdetect: Fallback to /proc to detect Linux

Check if /proc/sys/kernel/ostype reads as "Linux\n" to detect when a
"sysv" platform is really Linux. This test is a fallback with lower
priority than the existing heuristics based on ELF headers.

This fixes a problem with Alpine Linux docker images where LuaRocks
does not detect Linux because /bin/sh ELF headers don't contain have
the expected clues:

    $ docker run -ti alpine
    / # apk add --no-cache build-base
    [...]
    / # readelf -h /bin/sh
    ELF Header:
      Magic:   7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
      Class:                             ELF64
      Data:                              2's complement, little endian
      Version:                           1 (current)
      OS/ABI:                            UNIX - System V
      ABI Version:                       0
      Type:                              DYN (Shared object file)
      Machine:                           Advanced Micro Devices X86-64
      Version:                           0x1
      Entry point address:               0xc410
      Start of program headers:          64 (bytes into file)
      Start of section headers:          803616 (bytes into file)
      Flags:                             0x0
      Size of this header:               64 (bytes)
      Size of program headers:           56 (bytes)
      Number of program headers:         7
      Size of section headers:           64 (bytes)
      Number of section headers:         22
      Section header string table index: 21

1 files changed, 9 insertions, 0 deletions

diff --git a/src/luarocks/core/sysdetect.lua b/src/luarocks/core/sysdetect.lua
index bd5139b7..213d16bf 100644
--- a/src/luarocks/core/sysdetect.lua
+++ b/src/luarocks/core/sysdetect.lua
@@ -180,6 +180,15 @@ local function detect_elf_system(fd, hdr, sections)
             idx = idx + (vn_next * (vn_cnt + 1))
          end
       end
+
+      local procfile = io.open("/proc/sys/kernel/ostype")
+      if procfile then
+         local version = procfile:read(6)
+         procfile:close()
+         if version == "Linux\n" then
+            return "linux"
+         end
+      end
    end
    
    return system