New ixemul.library V50 for 68k AmigaOS

• Fix a bug in lseek, so ffmpeg work without changes
  
• Add atan2, strtoll func.Need link with new libc
  
• Over 25 Math C99 functions add in math.h
  
• add stdint.h and inttypes.h to includes.many programs need them
  
• add func atoll to stdlib.h

• autoconfig detects now correctly that AOS have a vfork function and uses this automatically
  (help gcc, make build and I guess lots other without source changes)
  
• assert use the in newer GCC obsolete eeprintf func not.
  
• Use V48 signal method this get working CTRL+c, OWB, vfork on 68k again(was broken in V49 68k)
  
  
• if no int __stack variable is set in program source, a default stacksize from the envvar ixstack is use.
  if no envarc:ixstack env var is find, the minimum stacksize is set to 512000 bytes.
  If on shell or Icon more stack than the minimum is set, then this stack size is use.
  
  Sound large, but Unix/Linux programs need lot stack.the value of 512 kb is a praxis value, because I find no program currently that use more stack than 400 kb.but don't think 512 kb is far enough.
  
  The cygwin Unix API give Unix/Linux tasks 2 MB of stack and also windows programs get on default 2.5 MB of stack. So if you have lots of ram and you want be on the safe side better set ixstack to 2000000
  
  If you are low on amiga mem, you can lower the stack value up to 16384 same value as ixemul 48 or 49.
  
  BUT BE SURE ALWAYS look how much stack a program need. There are not many programs out, that work correctly with this low stacksize. So you need carefully set the values in shell-startup or icons by yourself to useful values.
  
  
  
• An env variable IXENVARC added, if set then ixemul read the env variables on dir envarc and not in env: This "Hack" helps make it easy to get ixemul working when envhandler or happyenv is used.
  
  Change an envarc that already exists, work in env: and envarc
  
  Limits: when you create a new env variable only in env: and not in envarc: then this is not notice. So rule is, add new env vars always in envarc.
  
  
• stack usage can see when program end, work also on vfork tasks now In ixprefs enable show stack usage and start program ixstack -s from shell.